1. Field of the Invention
The present invention relates to a hydraulic control system for an injection molding machine, particularly to a hydraulic control system including a driving motor for a fixed delivery pump which is capable of supplying a hydraulic fluid at a rate corresponding to the driving speed of an hydraulic actuator included in the injection molding machine, and reducing a power consumption.
2. Description of the Related Art
FIG. 4 shows a conventional hydraulic control system for an infection molding machine. A fixed delivery pump 1, which is driven by an induction motor 2, draws the hydraulic fluid from a reservoir 3 and delivers it through a proportional flow control valve 4 to actuators included in the injection molding machine. The actuators are ones such as a clamping cylinder 5, an injection cylinder 7 for driving a screw 6 to move forward and backward and a charging hydraulic motor 8 for driving the screw 6 for rotation. A solenoid valve 9 is operated to control the claimping cylinder 5 for a closing and opening operation for the mold. A solenoid valve 10 is operated to control the injection cylinder 7, which advances the screw 6 for an injection operation and to retract the screw 6 for a charging operation. A flow control valve 11 regulates a hydraulic fluid supplied to the charging hydraulic motor 8. A solenoid valve 12 is closed to suspend the supply of the hydraulic fluid to the hydraulic motor 8. The hydraulic pressure in the hydraulic system is determined by a pressure relief valve 13 as desired. The proportional flow control valve 4 controlls the flow of the hydraulic fluid on the basis of an electric signal S given thereto by a main controller 14 for the injection molding machine so as to supply the hydraulic fluid at a flow rate necessary for driving each of the actuators at a desired speed.
The operation of the conventional hydraulic control system will be described hereinafter.
When clamping the mold in a clamping process, the proportional flow control valve 4 regulates the hydraulic fluid discharged from the fixed delivery pump 1 to flow through the solenoid valve 9 to the clamping cylinder 5 at a low flow rate which is predetermined by the controller 14. Initially, a movable platen, not shown, starts closing the mold at a low speed. Then, the proportional flow control valve 4 is controlled so as to supply the hydraulic fluid at a predetermined high flow rate to the mold clamping cylinder 5. In a final stage of the mold clamping operation, the hydraulic fluid is regulated to a predetermined low flow rate to close the mold gradually. After the mold has been fully closed, the hydraulic pressure in the clamping cylinder 5 is maintained to keep the mold clamped by a clamping force.
In a subsequent injection process, when the hydraulic fluid is supplied at a high flow rate to the injection cylinder 7, the screw 6 advances to inject a quantity of a molten polymer charged in front of the screw 6 into a cavity in the mold. After the injection process has been completed, a dwelling process is executed to prevent against a defect such as a sinkmark that occurs in a cooled polymer in the mold. In the dwelling process, the hydraulic pressure in the injection cylinder 7 is maintained until the polymer in the mold is cooled and solidified.
A mold opening process is started after the polymer in the mold has fully solidified. The movable platen is moved at a low speed during an initial stage and a final stage, but increased its moving speed during midway of a stroke of the mold opening. Finally, a molded part is ejected from the mold to complete the mold opening process.
In the mean time, the injection barrel is retracted from the mold, the screw 6 is rotated to charge the molten polymer for the next injection molding cycle. After a predetermined quantity of molten polymer is charged, the screw 6 is moved slightly backward for a melt decompression to prevent the molten polymer from drooling out of a nozzle attached to the injection barrel. Thus, one injection molding cycle is completed.
FIG. 5 shows another example of a conventional hydraulic control system for an injection molding machine. This hydraulic control system is provided with a large-capacity fixed delivery pump 17 and a small-capacity fixed delivery pump 18 with pressure relief valves 15 and 16, respectively, instead of the fixed delivery pump 1 as shown in FIG. 4. A solenoid valve 19 is controlled properly according to a desired flow rate. For example, when the clamping cylinder 5 is driven at a high speed, the solenoid valve 19 is opened so as to supply the hydraulic fluid with the clamping cylinder 5 from the both fixed delivery pump 17 and 18.
In the hydraulic control system shown in FIG. 4, the fixed delivery pump 1 is running so as to discharge continuously the hydraulic fluid at a maximum flow rate necessary to advance the screw 6. That leads to a power loss. For example, in the dwelling and cooling process in which a low flow rate should be supplied with the injection cylinder 7, surplus hydraulic fluid is returned through the pressure relief valve 13 into the reservoir 3. Thus, the fixed delivery pump 1, which is loaded continuously at the pressure set by the pressure relief valve 13, wastes a lot of power uselessly.
In the hydraulic control system shown in FIG. 5, a large flow of the hydraulic fluid is supplied with the clamping cylinder 5 through solenoid valve 19 by combining the delivery of pump 17 and 18. In a case that a small hydraulic flow should be delivered, the solenoid valve 19 is closed, and hydraulic fluid is delivered by the pump 18 with small capacity. However, the large capacity pump 17 is running continuously while the surplus hydraulic fluid is bent through the pressure relief valve 15. That is undesirable from the viewpoint of energy economy.